Variable valve engine timing has been used to accomplish the competing objectives of good fuel economy and wide open throttle acceleration over a significant portion of the engine speed range. To achieve variability of valve timing, numerous mechanisms have been proposed which typically provide a two-mode change-over between low and high speed engine operation. Recently, however, a mechanism has been described, Multi-Mode Variable Valve Timing Engine, Automotive Engineering/February 1994, p. 111-114 which provides multi-mode valve timing variability. This device has been stated to provide better engine output and fuel economy using a unique hydraulic mechanism for three-mode change-overs.
The device provides for the deactivation of both intake and exhaust valves, select low-speed cam with moderate lift and short duration and select high-speed cam with high lift and long duration. This device enables shutting off unnecessary cylinders during low-speed cruising or selecting optimum valve events during wide open throttle acceleration over the entire engine speed range. The modes are automatically selected by the engine control depending upon the engine's operating condition. The device consists of two different cams with low speed and high speed profiles, two rocker arms with roller followers, a valve actuating T-shaped lever with shaft, and two hydraulic pistons incorporated into the shaft. The hydraulic pistons are used to push and pull the shaft into low and high speed operation. While this mechanism achieves increased power output and fuel economy, it is very complicated in structure and operation. Other prior art devices exist which purport to provide variability in timing; however, they are also complicated and expensive to operate. Typically, these devices are used, if at all, on limited production or specialty engines.
Accordingly, it is an object of the invention to overcome the limitations and disadvantages of prior art devices and provide a reliable mechanism which achieves selectable valve timing over the entire engine speed range. It is a further object of the invention to provide a mechanism for selectable valve timing that is effective to increase engine output and fuel economy and is reliable. It is also an objective of the invention to provide a valve control mechanism for internal combustion engines which overcomes the complexity of the prior art devices and is simple to manufacture, implement, and use. It is still further object of the invention to provide a selectable valve timing mechanism that can be retrofitted onto existing internal combustion engines without major modifications of the existing engine structure. Finally, it is an object of the invention to provide a mechanism that is effective in apparatus requiring variability of valve-like activation.